Devices that measure deep-frying oil quality are known in which a deep-frying oil sample is taken from the deep fryer basin and the deep-frying oil quality is measured in a measuring instrument outside the deep fryer by means of a capacitor. This device takes advantage of the dielectric constant of the deep-frying oil, which changes as it ages. Using a correlation function, the polar fractions of the deep-frying oil can be calculated from the change of the dielectric constant.
It is furthermore known from DE 199 18 213 (now DE 100 15 516) that the quality of deep-frying oil can be measured with a device held in one hand. To carry out the measurement, the operator grasps the housing in which the electronic measurement system is arranged. On the underside of the housing, a metal pipe is fixed and an exposed interdigital capacitor is arranged on its free end. The capacitor is connected to the electronic measurement system with the electric wires running in the metal pipe and a part of this electronic system is also housed in the metal pipe. Apart from the capacitance (and thereby of the dielectric constant), the temperature of the deep-frying oil is also measured with the known device because the dielectric constant depends not only on the quality of the deep-frying oil, i.e. of the polar fractions, but also very much on the temperature of the deep-frying oil. The measured oil temperature enters directly in the calculation of the polar fractions. Therefore, the quantity of the polar fractions in the deep-frying oil—and thus the deep-frying oil quality—can be inferred directly from the measured values of the capacitance and temperature in the calculation of the polar fractions.
Moreover, it is known from U.S. Pat. No. 8,497,691 B1 that a stationary oil quality sensor is arranged in a filtering cycle, in a return flow or in a deep fryer tank. Like the manual measuring device mentioned above, the oil quality sensor has an interdigital capacitor.
The disadvantage in these two devices mentioned first is that they always demand the active participation of the operator. In the hectic environment of a fast food restaurant, however, the measuring task can often be forgotten. Also, the staff must handle the portable device rather carefully because otherwise the capacitor can be damaged if it touches the deep fryer basin, for example. Regarding its measuring precision, the known stationary oil quality sensor does not meet the highest requirements.